Sometimes the wieght of the blooms makes the stems flop but other times the plant is not getting enough moisture and it wilts in a manner similar as you describe. If the soil feels dry, I suggest watering more often in the summer months and making sure that the plant has 3-4" of mulch thru the drip line. If the soil is moist, you could prune the blooms if they are bending the stems a lot. They should get stronger in future years. Some support somtimes helps too.

My PD tree drooped severely in past years too. I think it is mostly from the weight of the blooms - especially after a lot of rain. This year I'm doing ok. Maybe because we've been fairly dry. Also could be because I removed at least half of the new growth in the early spring. Now the larger, more established branches are supporting less weighty new growth. And my blooms are quite large too. Next year I'm going to experiment more with a second pruning in mid-spring to keep the new growth a little shorter in some areas. I'd like to keep the canopy a little more symmetrical. We'll see how this works.

I pruned my limelight tree down to the second leaf last year and it has come back beautiful. I just got a pink diamond tree the other day and it's a little floppy but I think a good prune in early spring and it will be looking good. I have never seen one in the box stores or even the nursery look great. If you prune it good the second year will be much better.